15 gallon tall tank ideas

[swill]

So I've had 2 seahorses in a 15 gallon for quite some time now and one unfortunately died today from gas bubble disease. I've decided if the second one goes that I won't be trying to keep Seahorses again (which pains me because I love Seahorses.) I've starting searching the internet to see what else I can keep in the 15 gallon besides just coral and small fish. I'd like something different and am wondering if anyone has any good ideas. The tank has LEDs and a protein skimmer but no sump currently running in it.